The present invention relates to a composition for forming an electrolyte for a solid electrolytic capacitor, and a solid electrolytic capacitor. In particularly, the present invention relates to a composition for forming an electrolyte for a solid electrolytic capacitor by chemical oxidative polymerization of aniline, and a solid electrolytic capacitor produced by forming an electrolyte from the composition.
In the structure of an ordinary solid electrolytic capacitor, the anode comprises a tantalum pellet called a valve metal or an aluminum foil with an expanded surface, dielectrics comprising an oxide film formed on the surface of the anode, and an electrolyte layer formed on the dielectrics comprising manganese dioxide (MnO.sub.2), 7,7',8,8'-tetracyanoquinodimethane complex salt (TCNQ). However, since manganese dioxide has an insufficient conductivity of 0.1 S/cm, the solid electrolytic capacitor having the manganese dioxide electrolyte layer has a high impedance in a high-frequency zone and many layers of the manganese dioxide electrolyte which necessitates a high temperature must be formed by repeating the coating operation. Therefore, such a capacitor has an essential defect of leakage current. To overcome this defect, a reformation treatment for repairing the dielectric oxide film was necessitated after formation of each MnO.sub.2 layer to make the electrolyte-formation steps complicated. The solid electrolytic capacitor having the TCNQ electrolyte layer had only a poor thermal resistance because TCNQ is molten at a temperature of not higher than the soldering temperature. Further, since the conductivity of TCNQ is as low as about 1 S/cm, it could not satisfy the requirements of capacitors having more excellent high-frequency characteristics. Under these circumstances, solid electrolytic capacitors having an electrolyte layer comprising a conductive polymer having a thermal resistance superior to that of TCNQ were proposed. For example, Japanese Patent Unexamined Published Application (hereinafter referred to as "J. P. KOKAI") No. Sho 60-37114 discloses a capacitor having an electrolyte layer comprising a conductive polymer which is a doped polymer of a five-membered heterocyclic compound. J. P. KOKAI No. Sho 63-80517 discloses a capacitor having an electrolyte layer prepared by forming a thin layer by coating with a solution of a polymer of a five-membered heterocyclic compound in a volatile solvent and doping the formed layer.
However, the process for forming the electrolyte comprising the conductive polymer described in J. P. KOKAI No. Sho 60-37114 comprises complicated steps because it is an electrolytic polymerization process. Particularly the mass production of a small capacitor element such as that of a tantalum solid electrolytic capacitor was difficult. In addition, it is usually very difficult to conduct such an electrode reaction on the dielectric surface of the insulating capacitor. In the process described in J. P. KOKAI No. Sho 63-80517 wherein the solution of the conductive polymer in the volatile solvent in insulation state is used for coating, it is impossible to form a conductive polymer layer of a sufficient thickness in the capacitor element and, therefore, the thermal resistance of the capacitor is poor. Another defect of this process is that since the density of the conductive polymer film is excessively brittle to cause a serious change due to a stress in the steps, the properties thereof became poor after encapsulating it by molding.